\label{copyright}
In this section we give a brief introduction on the idea of copyleft licences. After that we explain the key facts in the history of copyleft licences and discuss the GNU General Public License (GPL) and Creative Commons (ShareAlike Attribute) as examples for copyleft licences.

The basic idea of copyleft licences is best described by the inventor of this kind of licences Richard Stallman:

\begin{quotation}
Copyleft uses copyright law, but flips it over to serve the opposite of its usual purpose: instead of a means for restricting a program, it becomes a means for
keeping the program free. \cite[page 13]{stallman2009free}
 \end{quotation} 
 
Copyleft ensures that free software stays free. This means that all published derivatives of the software have to be licensed under the same licence as the original work. Whereupon the definition for publishing differs from licence to licence. This claim is ensured by using the fundamental copyright of the author to bind the use of the software to the agreement to the licence. 

\begin{figure}
\begin{center}
\includegraphics[scale=.7]{images/copyleft_diagram} 
\end{center}
\caption{Idea of copyleft}\label{CopyleftIdea}
\end{figure}


In diagram~\ref{CopyleftIdea} the term 'more user rights' does not mean more rights for a single user but more rights for all users combined. That is because the users can be sure that the software will stay free. 

\nocite{gnuGPLfaq}



\subsection{History of Copyleft}

The term copyleft was first mentioned in a source code submission to the Dr. Dobbs Journal in 1976. It was not not bound to any licence but was intended as a statement against the copyright methods of that time \cite{dDobbs76}. 

The idea to create a copyleft licence --- that would be triggered upon using the software --- came from Richard Stallman. In 1988 he tried to force a company to publish their code changes after they used a program originally created by him. The program was in the public domain so he had no legal means to enforce his claim. This led him to develop the Emacs General Public License (specificity designed for the Emacs editor) in 1988 which soon became the GNU General Public License (applicable to every software) \cite{will02}. The GPL then became the most used FLOSS licence \cite{bdLicenceUsage}. 

After the success of the GPL there was demand for a copyleft licence for music, pictures and art in general. This led to the development of the Creative Commons licences. They were released in 2002 to give artists an easy way to make their creations available for a broad audience \cite{ccHist}. We focus on Creative Commons licences with the ShareAlike attribute because they are in the spirit of the copyleft idea. 


\subsection{Examples for Copyleft Licenses}

Copyleft licences are widely used in the open source community. Currently more than 50 percent of all open source projects use a copyleft licence \cite{bdLicenceUsage}. We focus on the GPL and its derivatives as examples for software copyleft licences and the Creative Commons Attribution-ShareAlike and Attribution-NonCommercial-ShareAlike as non software licences. These are the commonly used licences for their respective field. 

\subsubsection{GNU General Public License (GPL)}

The GPL is the most used copyleft licence. It is used in well known projects like the Linux Kernel, the Gnome Desktop Environment or the HotSpot Java Virtual Machine. When we talk about the GPL we actually refer to several licences. Which are the GPL, the LGPL, and the AGPL. These licences are designed to protect software products in very specific and specialized ways. 

All of them respect the four properties that characterize free software also known as \emph{Four Freedoms}.

\begin{enumerate}
\setcounter{enumi}{-1}
\item The freedom to use the software for any purpose
\item The freedom to change the software to suit your needs
\item The freedom to share the software with your friends an neighbors
\item The freedom to share the changes you make
\end{enumerate}

But also enforce that all published derivatives need to stay under the same licence. The licences differ in the definition of when a derivative is created or published. 

\begin{description}
\item[GPL (GNU General Public License)] The GPL is the basis for the LGPL and the AGPL. It is the oldest of the three and the most used. The licence is restrictive to the inclusion and usage of software under it. Therefore GPL software can not be included in not GPL compatible Software. And even linking to functions of GPL software is only allowed from GPL compatible Software.
\item[LGPL (GNU Lesser General Public License)] The LGPL is less restrictive than the GPL. It allows the linking from proprietary software but forbids the inclusion. It is usually used for software libraries that have proprietary counter parts. 
\item[AGPL (GNU Affero General Public License)]\label{AGPL} The AGPL is a more restrictive version of the GPL. It has a stronger definition for publishing a software. In the AGPL a software is considered published if either the software is given to a third party (as in the GPL and LGPL) or the functionality of the software is made available as a web service. The AGPL is therefore mostly used for software that has network capabilities.
\end{description}

\subsubsection{Creative Commons (ShareAlike Attribute)}

Creative Commons is the name of a foundation that develops several licences. These licences are build modular so that users can choose the attributes they see fit for their project. We only focus on licences that include the ShareAlike attribute, because this attribute is in the spirit of the copyleft idea. 

Creative Commons licences mainly focus on artistic works like music, pictures or texts.
 They have three layers (see Figure~\ref{CreativeLayers}) that all represent the same licence but are intended for different audience. The first layer is the machine readable layer. It is created in way that is easy distinguished by machines. It marks the work as licensed under a Creative Commons licence. The second layer is the human readable layer. It is intended to be read by humans. It describes the attributes of the licence in easy words. Even a layman can understand the rights and duties that come with the use of the work.  The third an last layer is the actual legal code of the licence. This code is the only thing that is actually legally binding. 

\begin{figure}
  \begin{center}
    \includegraphics[scale = .7]{images/three_layers}
  \end{center}
  \caption{The three layers of the Creative Commons. \\Creative Commons licence by Nathan Yergler and Alex Roberts}\label{CreativeLayers}
\end{figure}
The copyleft part of the Creative Common licences consists of the Attribution-Share\-Alike CC BY-SA and the Attribution-NonCommercial-ShareAlike CC BY-NC-SA licence. We are goning to give a general idea of these licences.

\begin{description}
\item[Attribution-ShareAlike 
\ccbysa
] The Attribution-ShareAlike licence allows the use of the work in any way if two terms are met. The original author of the work has to be \emph{attributed} and the derivative need to be \emph{shared} under the same licence. 
\item[Attribution-NonCommercial-ShareAlike
\ccbyncsa
] The Attribution-NonCommercial-ShareAlike licence demands that the work can \emph{not} be used in \emph{commercial} way in addition to the claims of the Attribution-ShareAlike licence. It could be argued, that not all \emph{Four Freedoms} are granted, because the use of the work is restricted.
\end{description}
\nocite{ccFAQ}

